Recently, WDM optical transmission systems that use wavelength division multiplexing (WDM) have become widespread. A WDM optical transmission system multiplexes and transmits a plurality of optical signals of different wavelengths. A reconfigurable optical add drop multiplexer (ROADM) is provided on a transmission line in the WDM optical transmission system. The ROADM branches (drops) the optical signals of a desired wavelength from the WDM signals and inserts (adds) the branched signals into empty channels in the WDM signals. The ROADM also includes an erbium-doped fiber amplifier (EDFA) for amplifying the optical signals.
An optical amplifier is provided in each node in order to compensate for loss in the transmission line and the ROADM in the WDM optical transmission system. The wavelength characteristics of the optical gain and the optical loss with respect to the WDM signal depend upon the wavelength allocation of the WDM signals. Therefore, a function for adjusting the optical power of each wavelength channel is provided in the WDM optical transmission system. This function is realized, for example, through an optical channel monitor (OCM) that detects the optical power in each wavelength channel and a wavelength selective switch (WSS) that adjusts the optical power in each wavelength channel. In this case, the optical power in each wavelength channel is controlled so that the optical power of an optical signal received by a receiving node is maintained within the receivable power range of the optical receiver.
When the wavelength allocations of the WDM signals are changed, the optical power in each wavelength channel temporarily fluctuates greatly thereby producing a gain ripple (variation of the optical gain with respect to the wavelength) in the EDFA. Consequently, the power of the optical signals arriving at the receiving node may beyond the receivable power limits of the optical receiver and an optical signal error may occur. This problem occurs when there is a large change in the optical gain wavelength characteristics caused by the changes in the wavelength allocation of the WDM signals.
One factor that leads to a gain ripple in the EDFA is spectrum hole burning (referred to below as SHB). SHB is produced when the optical signals pass through the EDFA. Specifically, when the optical signals pass through the EDFA, there is a decrease in the wavelengths of the optical signals and in the gain of the wavelengths in the proximity thereof.
Namely, the optical power in each wavelength channel fluctuates due to the changes in the shape (gain wavelength characteristics) of the gain ripple caused by the SHB following the changes in the wavelength allocation of the WDM signals. As a result, there is a possibility that an optical signal error may occur in a wavelength channel with a large fluctuation of the optical power.
Japanese Patent No. 4643645 proposes a technique which involves dividing the wavelength bands of the WDM signals into bands in which the optical power is changed based on the SHB when the number of signal wavelengths is low, and into other bands, and performing gain correction on the WDM signals based on the number of signal wavelengths in each band according to the result of monitoring the optical power in each band. Non-patent documents (1: C. Randy Giles, Emmanuel Desurvire, “Modeling Erbium-Doped Fiber Amplifiers”, Journal of Lightwave Technology, vol. 9, no. 2, pp. 271-283 (1991); 2: Maxim Bolshtyansky, “Spectral Hole Burning in Erbium-Doped Fiber Amplifiers”, Journal of Lightwave Technology, vol. 21, no. 4, pp. 1032-1038 (2003)) respectively describe the modeling of the EDFA and the phenomenon of spectral hole burning.